I am looking at the inside of a 3 prong outlet cord. (for a house with 120 volt ac )

It has 3 different color wires

white
black
and green

But realisticly why do we need three wires?
To complete a circuit you should only need 2 wires?

I have read that black and white wires are used in completing the house circuit and the green is only ever used when their is a faliure in the white/black circuit. (for protection)

But I don't really get how a third wire could protect you?
I have read that the green wire basically touchs the devices metal case.
So if the black wire comes undone and touches the metal case it will travel to the green ground wire instead of thru you.

But is this the only reason or is their more to the protection reason what happens if the device didn't have a metal case then would the green ground be completely worthless. Where would you even attach it?

Where is green in the house circuit connected to?
I know black is power and white is the return of the power.
I am assuming since it is ac it osscilates between black having current flowing to white and white having current flowing to black.

The issue of ground has been brought up many times in the Electronics Chat forum. Basically you need the two wires to energize the equipment, and the 3rd wire as a safety feature. They are Hot (black), Neutral (white), and Ground (green).

Take something like a drill motor plugged into the wall. If it is a two wire model (which used to be the standard) and the hot wire shorts to the metal case you have a very good chance of being injured by electrocution. If the metal case (aka chassis) is grounded with a third wire then you have a breaker blow, but you probably don't feel a thing.

Most appliances, such as drill shafts, have metal somewhere, so even a plastic case is no guarantee of protection.

The ground and the neutral are earthed at your line tap, where your house (or whatever) gets its electricity, with a deeply buried spike or wire in the ground.

The green is usually attached on metal parts of the device. If the hot wire touches accidentally the metal part the sensing device (GFI) located in the distribution box will brake the circuit.

Have a look in google for 'ground fault interrupter' for more information.

If you plug the multimeter in the socket between the black and white wires you will read 120VAC if the meter reads RMS values.
It is dangerous if you don't know you to use it. Be careful not to set the meter to measure current and plug it into the socket, it will be damaged or may explode. Also, take care not to put the metallic edge of the probes very close to each other because a spark will be created.

You measure 120VAC between black and green because the green wire is connected to the white wire in the distribution box. However, you are not allowed to run any application with these two wires because the GFI will trip.

The white "neutral" wire is the grounded conductor, the bare or green wire is the grounding conductor. The white is connected to the center-tap of the power company's transformer, and also to ground. (You should find a grounding electrode very close to your meter. Yes, that's a grounding electrode for the grounded conductor. ) The hot wires are connected to the ends of said transformer.

The hot wire can also be red, and will indeed be red on an American split phase 240V system. (A good explanation of the split phase system is found here: http://www.allaboutcircuits.com/vol_2/chpt_10/1.html) On three phase systems, one can also find a blue hot. Orange, brown, and yellow are used for 220V three phase hot leads.

Europe uses different colors entirely.

As has been previously noted, the colors are just colors and may not have been installed properly. Always exercise caution when dealing with lethal voltages!!

If you stick the hot wire in your mouth while you are grounded, current will flow back and forth (AC) through you. This is because your mouth will be at the same potential as one side of the power company's transformer (120VAC to ground) and your feet will be at ground potential.

If you stick the grounded wire in your mouth while you are grounded, your mouth and feet will both be at ground potential. Do NOT try this at home.

The colors are nothing more than handy visual indicators, and then only if they have been installed correctly.

Look at it this way: the AC power is flowing thru the hot wire "back and forth" like a snake, just like you are envisioning. White is 'at rest' until you plug something into the outlet, at which time it completes the path to ground and the device is then 'on'. If you disconnect the 3 wires, the only one that will have AC will be the black one.
PROVIDED the circuit is wired correctly (and trust me, MANY times they are not; someone always has to wire 1 outlet wrong and get everything backwards!). Rather than look at AC as 'power going back and forth', it's easier to think of it as just simply reversing/inverting along that 1 hot wire.

The safety ground (green terminal, bare copper generally) is a redundant ground connection that keeps all the junction and outlet boxes at the same ground potential regardless of what's going on with the 2 'power carrying wires'. As above, if there is a short to the box, the ground wire takes on the job of the white wire. It's actually very handy, since 1 bad connection can really ruin your day otherwise! It acts like the overflow slot in a sink that prevents the sink from spilling over...doesn't do anything until the situation arises, but if it wasn't there, you would regret it!

Another great scenario is when someone connects hot to ground - or there is a ground fault somewhere - and you get power running on the ground at a current not high enough to trip the breaker/fuse...then you can have a sink or water pipe that will shock you! I've even gotten bit by a damp wooden floor joist. But that's another story...

There are several acceptable ways, but this is the way I wired my house:

The Romex from the breaker panel is led to the wall box with the switch. The Romex is a trade name for a non-metallic cable containing two insulated conductors and a bare wire. This would be referred to as 14/2 with ground. The cable superinsulates the internal conductors. One insulated wire is black; the other white. The black wire attaches to the branch circuit breaker, and carries AC hot @ a nominal 120 VAC. The white attaches to the common bus, as does the bare copper ground wire.

Also present in the wiring box is Romex from the light fixture. The fixture has pigtails in place, one black and one white, plus a green screw to capture the ground wire. Attachments to the pigtails are done with wire nuts.

In the box proper, the ground wires are connected, with a short bare pigtail that runs to the ground lug on the switch. The whites simply are connected with a wire nut. The supply black is stripped to length and looped around one switch captivating screw. The black from the fixture is run to the other switch attaching screw.

Mount the switch to the box, place the cover over all to hide the hole and mess, and there it is.

Beenthere's description is correct and proper for non-metallic boxes. When using a metal box, the box must also be bonded to the grounding (green or bare) conductor.

The reason for this is simple: it is just another metal case with electrical stuff inside it. If the hot wire inside comes loose and touches ungrounded metal, it presents a danger to anyone crawling in the attic or nailing/drilling into the wall from the other side. If it touches grounded metal, the breaker trips and someone has to find and fix the problem.

The Romex from the breaker panel is led to the wall box with the switch

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I don't doubt you for one instant. I am simply trying to understand US wiring practice.

What you are describing is a simple series arrangement. The neutral (white) is jointed but passes through the box unswitched. The hot (black) is switched by a series connected single pole switch on the switchplate.

Since the supply comes direct from the breaker panel, do you wire a separate cable from the breaker to each switch box in each room, or do you use a 'loop in loop out system' like us?

Curent UK practice is to use the 'loop in loop out' system whereby a single cable containing live, neutral and bare earth starts at the breaker and passes to the first ceiling rose or box, not the room switch box. This is the 'loop in'. The cable then 'loops out to the next ceiling box and so on to the last one. No return ring connection is made to the breaker panel.

So the ceiling box always contains an unswitched live terminal.

In fact the ceiling box contains four poles.

The first pole is the loop in/loop out live and the live feed to the switchbox.
The second is the loop in/loop out neutral and the neutral to the pendant wire for the bulbholder.
The third is the live feed from the switch and the live to the pendant wire.
The fourth is the loop in loop out earth.

So the feed to the switch, a single cable containing a live feed, a second conductor in the same colour as the neutral and a bare earth goes down to the switchbox from the ceiling box.

At the switchbox the two conductors are connected to either side of the single pole switch and the earth conductor earths the box. No neutral is connected to or in the switchbox at all.

This means that one side of the switch is always live and the correct colour for live (formerly red, now brown) but the other side of the switch is connected to a wire normally associated with neutral (formerly black, now blue). To indicate that this is actually a live or hot wire (when switched on) this wire is 'designated' red (brown) and the electrician's apprentice is meant to stick a small ring of red (brown) tape around it to show this.

The U.S. practice is to divide feeds from each breaker, with attention to keeping each branch at a safe maximum current. A 15 amp circuit should draw no more than 12 amps and is wired with 14 ga. A 20 amp circuit is limited to 16 amps and uses 12 ga.

We can loop, or feed the wiring through a series of switches - provided the volume of the box is sufficient to allow for the extra connections. In many cases, it is simpler and uses less wire to divide the feed in a 4" or 6" metallic box. The leads are fed out to several switches, again with attention paid to the maximum allowable number of joints per the volume of the box. When you are dealing with multiple switches in one box, it gets crowded in a hurry, no matter the depth.

Wall outlets are often "looped", or fed through with a set of joints and pigtail to connect to the outlet in each box.

Rather than look at AC as 'power going back and forth', it's easier to think of it as just simply reversing/inverting along that 1 hot wire.

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Wow, I feel stupid now because I also always thought that an AC outlet's 2 leads were equivalent. So are you saying that an AC outlet only needs one of its connections to be fed directly from the power supply. In other words, is power supplied from the power company's generator using only one wire?

Lets say you took a light bulb and connected one lead to the AC outlets hot wire, and the other lead you just connected to a metal rod in the ground, would it work (if you also disconnected all safety features in the houses wiring)? If it would work is there any loss of efficiency in this setup?

No, not at all - look at the number of wires entering your service drop. Two hot phases and one common. All power is distributed over multiple wires. Observe any power line for confirmation.

The lamp would light up, but conduction through the actual ground is affected strongly by moisture and such. Not at all efficient, or it would be done that way. And not at all safe. Your common and ground at the outlet are already referenced to ground - just confine power to the plug. You make a good enough conductor to get a fatal shock by being in contact with AC hot and any grounded surface.

Okay, now I'm really confused, so I'll try to simplify my question. If we just look at an AC generator, like a motor with a hand crank, there are two leads out of it right? Wouldn't both of those leads be "hot" (i.e. have current oscillating at some frequency)?

Okay, now I'm really confused, so I'll try to simplify my question. If we just look at an AC generator, like a motor with a hand crank, there are two leads out of it right? Wouldn't both of those leads be "hot" (i.e. have current oscillating at some frequency)?

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You can determine either of the two wires to be the hot wire. The wire which will be determined as the neutral will be connected to ground too via the ground wire. The other wire (not grounded) will be the hot wire.